The fundamental measurements in gait analysis consist of joint motion and net joint kinetic estimates. The fact that such measurements are referenced to joints implies knowledge of the positions of the skeleton. Data on skeletal position are typically obtained by measuring the locations of targets attached to the surface of the body. Errors in these data are introduced by the movement of the targets relative to the skeleton, due to the combined effect of the target attachment technique and displacements of the skin surface relative to the bone. The magnitude and pattern of these "surface movement errors" in standard gait data remain unknown, due to difficulties in measuring the true skeletal motion. Available data suggest that surface movement errors may be of considerable magnitude. The purpose of this project is to investigate the magnitudes and patterns of the surface movement errors during walking. A principle objective necessary to achieve this goal was to develop and demonstrate a minimally invasive technique for making the measurements.